WikiLeaks founder Julian Assange is suffering from “intense psychological trauma” as he serves a British jail sentence, and his human rights could be seriously violated if he’s extradited to the U.S., the United Nations’ special rapporteur on torture said.

Assange is suffering from extreme stress and chronic anxiety, Nils Melzer, who visited him in jail this month, said Friday in a statement. If transported to the U.S. he may face a life sentence or possibly the death penalty if new charges are added, Melzer said, urging the U.K. government not to send him there.

The comments come after Assange’s attorney said her client was unwell when the Australian failed to show up for a London court hearing on Thursday in his extradition battle. The 47-year-old is serving a 50-week sentence in the U.K.’s Belmarsh jail for skipping bail and has been moved to the prison’s health ward, according to WikiLeaks.

The U.S. last week charged Assange with 18 counts related to endangering national security by conspiring to obtain and disclose classified information, a move that escalated its extradition attempt.

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He’s accused of working with former U.S. Army intelligence officer Chelsea Manning to get classified documents from databases containing about 90,000 Afghanistan-war-related activity reports, 400,000 Iraq-war-related reports and 250,000 State Department cables.

Swedish prosecutors have separately reopened investigation into rape allegations against Assange, which he denies.

In the U.S., the Australian national’s rights — such as freedom of expression, the right to a fair trial and the prohibition of torture — could be violated, Melzer said in the statement. He lacks access to case files and has limited time with lawyers, making it “impossible for him to adequately prepare his defense” in the lawsuits, he said.

Assange “chose to hide in the embassy and was always free to leave and face justice,” U.K. foreign secretary Jeremy Hunt tweeted Friday. “The UN Special Rapporteur should allow British courts to make their judgments without his interference or inflammatory accusations.”

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The Australian government said in a statement that Assange was being treated appropriately in the U.K.’s Belmarsh prison, and that the country was providing him “active and high level” consular help. It’s asked Belmarsh about his health after reports on Thursday that he’s unwell, it said.